Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Polymeric membranes have proven to operate successfully in industrial gas separations such as in the separation of nitrogen from air and the separation of carbon dioxide from natural gas. Cellulose acetate (CA) commercial spiral wound and hollow fiber membranes have been used extensively for natural gas upgrading. However, CA membranes still need improvement in a number of properties including selectivity, performance durability, chemical stability, resistance to hydrocarbon contaminants, resistance to solvent swelling, and resistance to CO2 plasticization. Natural gas often contains substantial amounts of heavy hydrocarbons and water, either as an entrained liquid, or in vapor form, which may lead to condensation within membrane modules. The gas separation capabilities of CA membranes are affected by contact with liquids including water and aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX). The presence of more than modest levels of liquid BTEX heavy hydrocarbons is potentially damaging to the CA membrane. Therefore, precautions must be taken to remove the entrained liquid water and heavy hydrocarbons upstream of the membrane separation steps using expensive membrane pretreatment system. Another issue of CA polymer membranes that still needs to be addressed for their use in gas separations in the presence of high concentration of condensable gas or vapor such as carbon dioxide (CO2) and propylene is the plasticization of the polymer by these condensable gases or vapors that leads to swelling of the membrane as well as a significant increase in the permeance of all components in the feed and a decrease in the selectivity of CA membranes. For example, the permeation behavior of CO2 in CA membranes is different when compared to some other glassy polymers in that above a certain pressure level, the permeability coefficient begins to increase with pressure due to the onset of plasticization by the CO2. A high concentration of sorbed CO2 leads to increased segmental motion, and, consequently, the transport rate of the penetrant is enhanced. The challenge of treating gas, such as natural gas, that contains relatively large amounts of CO2, such as more than about 50%, is particularly difficult.
In addition, some natural gas feed has high CO2/C2+ concentration (usually CO2>70%). Membranes can be used to recover the high value natural gas liquid while removing CO2 from natural gas. Membranes can separate CO2 from CH4 and C2+ and recover C2+ from the membrane retentate. When using membranes for this separation, the feed side temperature drops significantly due to CO2 permeation (J-T effect), and the feed gas dew point increases as CO2 permeates, therefore liquid comes out from membrane system. The membranes, however, show significantly decreased membrane permeance in the presence of liquid aliphatic hydrocarbons, liquid aromatics, or both liquid aliphatic hydrocarbons and liquid aromatics.
Therefore, new robust membranes with stable performance under repetitive short term exposure to liquid hydrocarbon condensation, high resistance to hydrocarbon contaminants, high resistance to solvent swelling, and high resistance to CO2 plasticization desired for natural gas upgrading.
Polymeric membrane materials have been found to be of use in gas separations. Numerous research articles and patents describe polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyethers, polyamides, polyarylates, polypyrrolones) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (see, for example, U.S. Pat. No. 6,932,589). The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The membrane performance is characterized by the flux of a gas component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a gas mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity. Selectivity can be defined as the ratio of the permeabilities of the gas components across the membrane (i.e., PA/PB, where A and B are the two components). A membrane's permeability and selectivity are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent. It is desired to develop membrane materials with a high selectivity (efficiency) for the desired component, while maintaining a high permeability (productivity) for the desired component.
The relative ability of a membrane to achieve the desired separation is referred to as the separation factor or selectivity for the given mixture. There are however several other obstacles to use a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions. One such obstacle is permeation rate or flux. One of the components to be separated must have a sufficiently high permeation rate at the preferred conditions or extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Therefore, commercially available polymer membranes, such as CA, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. See U.S. Pat. No. 3,133,132. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. Such membranes have a serious shortcoming in that, in operation, the permeation rate and/or selectivity is reduced to unacceptable levels over time. This can occur for several reasons. One reason for the decrease of permeation rate has been attributed to a collapse of some of the pores near the skinned surface of the membrane, resulting in an undue densification of the surface skin. One attempt at overcoming this problem has been the development of thin film composite (or TFC) membranes, comprising a thin selective skin deposited on a resilient porous support. See, for example, “Thin-Film Composite Membrane for Single-Stage Seawater Desalination by Reverse Osmosis” by R. L. Riley et al., Applied Polymer Symposium No. 22, pages 255-267 (1973). TFC membranes can be formed from CA, polysulfone, polyethersulfone, polyamide, polyimide, polyetherimide, cellulose nitrate, polyurethane, polycarbonate, polystyrene, etc. While TFC membranes are less susceptible to flux decline than phase inversion-type membranes, fabrication of TFC membranes that are free from leaks is difficult, and fabrication requires multiple steps and so is generally more complex and costly. Another reason for the reduced permeation rate and/or selectivity over time is that impurities present in the mixture can over time clog the pores, if present, or interstitial spaces in the polymer. Yet another reason is that one or more components of the mixture can alter the form or structure of the polymer membrane over time thus changing its permeation rate and/or selectivity. One specific way this can happen is if one or more components of the mixture cause plasticization of the polymer membrane. Plasticization occurs when one or more of the components of the mixture act as a solvent in the polymer often causing it to swell and lose its membrane properties. It has been found that polymers such as cellulose acetate and polyimides which have particularly good separation factors for separation of mixtures comprising carbon dioxide and methane are prone to plasticization over time thus resulting in decreasing performance of these membranes.
One approach to overcoming the problem of leaks in asymmetric integrally skinned membranes has been the fabrication of an asymmetric integrally skinned gas separation membrane comprising a relatively porous and substantial void-containing selective “parent” membrane such as polysulfone or cellulose acetate that would have selectivity were it not porous, wherein the parent membrane is coated with a material such as a fluoropolymer, polysiloxane, a silicone rubber, or a UV-curable epoxysilicone in occluding contact with the porous parent membrane, the coating filling surface pores and other imperfections comprising voids (see US 20090277837 A1, U.S. Pat. Nos. 4,230,463; 4,877,528; 6,368,382). U.S. Pat. No. 4,230,463 provides one of the first examples for using a silicone rubber coating material to improve the selectivity of a polysulfone gas separation membrane. The coating of such coated membranes comprising siloxane or silicone segments, however, is subject to swelling by solvents, poor performance durability, low resistance to hydrocarbon contaminants, and low resistance to plasticization by the sorbed penetrant molecules such as CO2 or C3H6. On the other hand, delamination occurs easily for the coating layer of hydrophobic fluoropolymers directly on the relatively porous and substantial void-containing selective “parent” polymer membrane in the presence of liquid aromatic hydrocarbon contaminants such as BTEX in the gas feed.
Therefore, an asymmetric membrane post-treatment is needed which improves selectivity, resistance to liquid hydrocarbon contaminants, high resistance to solvent swelling, and high resistance to plasticization, but does not change or damage the membrane, or cause the membrane to lose performance with time. In addition, gas separation membranes desirably have a high permeation rate to gases. This means that the effective portion of the membrane should be as thin as possible. Therefore, the coating layer on the top surface of the relatively porous and substantial void-containing selective “parent” asymmetric membrane needs to be thin and the materials used as the coating layer should not significantly reduce the membrane permeance or flux.
The present invention discloses a new dual layer-coated asymmetric membrane for gas separations wherein the membrane has a relatively porous and substantial void-containing selective asymmetric membrane support, a first layer comprising a hydrogel coating and a second layer comprising a hydrophobic fluoropolymer coating. The relatively porous and substantial void-containing selective asymmetric membrane support can be made from any polymeric material such as polysulfone, polyethersulfone, polyimide, polyetherimide, cellulose acetate, cellulose triacetate, and mixtures thereof, and the hydrogel coating can be formed from a water-soluble polymeric species capable of forming a hydrogel such as gelatin and sodium alginate. The new dual layer-coated asymmetric membranes have advantages of low cost, high permeance (or flux), as well as stable permeance (or flux) and sustained selectivity over time by resistance to solvent swelling, plasticization and liquid hydrocarbon contaminants for gas separation applications.